Silver halide photothermographic imaging materials, especially "dry silver" compositions, processed with heat and without liquid development have been known in the art for many years. Such materials are a mixture of light insensitive reducible silver source material, a minor amount of light sensitive silver halide in catalytic proximity to the silver source material, and a reducing agent for the silver source.
The silver source material is a material which contains silver ions. The earliest and generally preferred silver source materials comprise silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have primarily been used.
The light sensitive silver halide is in catalytic proximity to the light insensitive silver salt such that the latent image formed by the irradiation of the silver halide serves as a catalyst nucleus for the oxidation-reduction reaction of the organic: silver salt with the reducing agent when heated above 80.degree. C. Typically, the reducing agent is a colorless developer or else is a lightly colored leuco dye or dye forming developer that is oxidizable to a colored state. Such developers in photothermographic media are described in U.S. Pat. Nos. 3,457,075; 3,839,049; 3,985,565; and 4,260,667.
Multicolor photothermographic imaging articles typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic articles having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747 and Research Disclosure 29963.
A common problem that exists with these photothermographic systems is the instability of the image following processing. The photoactive silver halide still present in the developed image may continue to catalyze print-out of metallic silver even during room light handling causing a strong increase of fog after development. This is also increased by the presence of oxygen in the air which causes the oxidation of leuco dyes. For example, U.S. Pat. Nos. 4,670,374 and 4,889,932 describe photothermographic materials containing oxidable leuco phenazine, phenoxazine or phenothiazine dyes useful to give colour photothermographic images. Unfortunately they are subjected to aerial oxidation, which causes increasing fog after development.
Another problem is the lack of stability of the leuco dyes before exposure: in fact, in many cases, it is not possible to obtain any images because the leuco dye reacts in a non-image-wise way before exposure. The consequence of this non-image-wise reaction is the absence of sensitometric effects. This means that there is no difference in the print-out between the parts that should have produced an image and the parts that should not have produced any image. European Patent Application No. 35,262, and PCT Patent application No. WO 90-00,978 describe, respectively, non-silver copy materials and non-silver heat-sensitive materials both having leuco dyes with the same --SO.sub.2 -- protecting group. These leuco dyes are useful in heat-sensitive materials. They are not useful in photothermographic materials because they do not react image-wise to give a dye image. In fact, when the material containing such leuco dyes is exposed and developed according to the usual process for photothermographic materials, it does not present any sensitometric effects.
Thus, there exists a need to have useful leuco dyes for photothermographic materials which are stable enough not to be aerially oxidised or by simple heating, and which limit fog formation after development to the simple print-out due to the presence of photosensitive silver halide. They also must react image-wise to provide a good dye image.
British Patent No. GB 1,417,586 describes the preparation of oxichromic compounds containing a reduced azomethine linkage. Such compounds produce upon chromogenic oxidation a chromophore useful in colour photographic systems, particularly in silver halide transfer materials. These oxichromic compounds may have a group which prevents oxidation of the N atom of the azomethine linkage and which hydrolizes off in alkaline solution and, in addition, they have a hydroquinone moiety in their structures. They are hence different from the compounds of the present invention and are used for a different purpose.